Method for the production of single crystals

ABSTRACT

An improved arc-fusion method for the production of high-quality single crystals at comparatively low cost. An arc is established in a first refractory sinterable powder pack to form a first internal cavity. A port is formed in the wall of the pack, and a second powder pack is formed within the cavity, this second pack consisting of a selected refractory sinterable powder from which single crystals are to be obtained. An arc is established in the second pack to melt only an internal portion thereof, forming a second internal cavity. Molten material derived from the second pack collects in the second cavity, cooling and crystallizing therein. A port is formed for withdrawal of the crystallized material, from which single crystals subsequently are harvested. The second cavity may be so used in additional crystal-growing runs.

United States Patent [72] lnventors Charles T. Butler;

Bernard J. Sturm, both of Oak Ridge, Tenn. [21] Appl. No. 45,087 [22] Filed June 10, 1970 [45] Patented Jan. 11, 1972 [73] Assignee The United States of America as represented by the United States Atomic Energy Commission [54] METHOD FOR THE PRODUCTION OF SINGLE CRYSTALS 9 Claims, 5 Drawing Figs.

[52] U.S. Cl 23/201, 23/186, 23/304 [51] Int. Cl C0lf5/02, COlf 1 1/02 [50] Field of Search 23/201, 295, 304, 110 M; 204/61 [56] References Cited UNITED STATES PATENTS 2,711,435 6/1955 Humphrey 23/110M 3,251,659 5/1966 Muelleretal ABSTRACT: An improved arc-fusion method for the production of high-quality single crystals at comparatively low cost. An arc is established in a first refractory sinterable powder pack to form a first internal cavity. A port is formed in the wall of the pack, and a second powder pack is formed within the cavity, this second pack consisting of a selected refractory sinterable powder from which single crystals are to be obtained. An arc is established in the second pack to melt only an internal portion thereof, forming a second internal cavity. Molten material derived from the second pack collects in the second cavity, cooling and crystallizing therein. A port is formed for withdrawal of the crystallized material, from which single crystals subsequently are harvested. The second cavity may be so used in additional crystal-growing runs.

Pmmwmn *1 ma INVENTORS. Ies T Butler 0rd J. SIurm Char Bern ATTORNEY.

METHOD FOR THE PRODUCTION OF SINGLE CRYSTALS BACKGROUND OF THE INVENTION This invention was made in the course of, or under, a contract with the United States Atomic Energy Commission.

This invention relates generally to the production of single crystals by submerged-arc fusion of a refractory powder, and more particularly to an improved method for producing such crystals.

As is well-known, single crystals of various refractory materials can be grown by the method referred to as submerged-arc fusion. As illustrated in terms of magnesium oxide, a conventional form of that process comprises the following principal operations. First, powdered magnesium oxide is packed about the tips of a plurality of converging electrodes. Second, an arc is established between the electrodes for a time sufficient to melt a selected fraction of the powder pack and form a roughly spherical cavity bounded by a glassy, nonporous wall; a melt of crystals collects in the bottom of this cavity. Third, after cooling and crystallization of the melt, the powder pack surrounding the cavity is broken away and the crystals harvested. Because of pickup of impurities from the electrodes and elsewhere, little or none of the powder pack remaining after melting is suitable for subsequent use as the melt material.

Submerged-arc fusion is an attractive method for the growth of crystals because the refractory powder charge serves as its own crucible, eliminating the problem of contamination of the melt by its container. Unfortunately, however, the method has an inherent limitation: to obtain good yields of even small single crystals it is essential that the mass of powder pack serving as an insulator and containment vessel be several times that of the powder to be melted.

The necessity of using large amounts of the powder for containment, coupled with the fact that the powder used for containment is not used in subsequent runs as the melt material, has imposed a severe economic penalty on the production of single crystalsparticularly where the crystals are to be grown from powders which are highly valuable because of their rarity, extreme purity, isotopic enrichment, doping, or the like. Assume, for example, that conventional submerged-arc fusion is being considered for the production of large single crystals from a powder pack consisting of highly pure magnesium oxide costing about $1,000 a kilogram. A total powder mass of about 60 Kg is required for a yield of about 5 kg. of the product crystals. Thus, considering powder costs alone, the cost of the product is prohibitive-about 12,000 a kilogram.

SUMMARY OF THE INVENTION It is, therefore, an object of this invention to provide an improved method for the production of single crystals by submerged-arc fusion.

It is another object to provide a low-cost method for growing single crystals by submerged-arc fusion.

It is another object to provide a method for producing single crystals by submerged-arc fusion of a highly valuable powder, the total amount of such powder required being small compared to that employed in conventional submerged-arc fusion process for producing such crystals.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 through 5 are schematic diagrams illustrating successive changes effected in an electrode-and-powder-pack arc fusion assembly in the course of a crystal-production run conducted in accordance with this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT For brevity, our improved method will be discussed primarily in terms of a typical application: the production of highly pure, large single crystals from a high-purity powder consisting essentially of magnesium oxide (Mg0) whose unit cost is much higher than that of less-pure commercial-grade magnesium oxide powder.

Referring to FIG. 1, the first step of our method comprises forming a powder pack 1, consisting essentially of commercial-grade magnesium oxide, about the converging tips of a plurality of standard consumable electrodes 2. In accordance with conventional submerged-arc fusion technology, the mass of this powder pack is preselected to be at least several times that of the portion of the pack to be melted After the powder pack 1 is formed as described, an arc is established between the electrodes 2 to initiate a powdermelting operation. This operation is conducted in the conventional fashion to melt a selected fraction of the powder pack 1. As shown in FIG. 2, this operation produces a roughly spherical cavity 3, which is bounded by glassy, nonporous walls and whose bottom portion contains a melt 4 of the magnesium oxide.

The melt 4 is permitted to cool and crystallize, after which a portion of the powder pack 1 surrounding the cavity is carefully removed to form a port 5, shown in FIG. 3. The cooled mass of crystals is withdrawn through this port in large pieces; if desired, individual crystals subsequently are harvested from these pieces. The electrodes 2 are withdrawn from the powder pack to facilitate removal of the crystalline mass and to permit the installation of unspent electrodes 2' (FIG. 4).

After withdrawal of the crystals and replacement of the electrodes, the cavity 3 and port 5 are packed with powder consisting essentially of magnesium oxide having much higher purity than the powder used to form pack 1. The resulting second, or inner, powder pack is designated as 6 in FIG. 4.

A second melting operation now is initiated by establishing an arc between the electrodes 2'. For this operation the melting parameters (electrical power input and period of melting) are adjusted to limit melting to an internal portion of the second powder pack 6. As indicated in FIG. 5, this operation produces a second roughly spherical cavity 7, which is encompassed by a continuous shell 8 of unmelted more-valuable oxide and which contains a melt 9 of more-valuable oxide. The shell 8 is provided to serve as a barrier preventing contamination of the melt 9 by the less-pure powder composing the first, or outer, powder pack 1. The Mg0 shell 8 must withstand melting, but thickness of 1 cm. or even less meet this requirement.

Following the second melting operation (FIG. 5), the cooled mass 9 of valuable-oxide crystals is selectively recovered from the powder packs. Preferably, this is accomplished by carefully forming a port (not shown) in the compacted-powder wall to permit removal of the crystalline mass while leaving the containment vessel intact. The cavity 7 then may be repacked with a fresh charge of the more-valuable oxide in preparation for another crystal-growing operation of the kind described in connection with FIGS. 4 and 5. Additional runs of this kind may be conducted indefinitely with charges of MgO of the same type used in the second melting operation, so long as the process parameters (time and electrical power input) are controlled to prevent violation of the above-mentioned shell 8.

EXAMPLE I First Melting Operation Magnesium oxide single crystals characterized by high purity and a high degree of optical clarity were prepared n accordance with this invention, using a conventional watercooled, three-electrode, three-phase submerged-arc fusion furnace. The furnace included a water-cooled stainless steel tub formed with a depression, or receiver, for containment of the powder charge. The consumable electrodes, which were composed of graphite, were mounted at an angle of 42 to the vertical and were positioned to converge at a point 25 cm. above the bottom of the receiver. Before installation, the various electrodes were bored axially to provide passages for introducing a sweep gas (argon, air, helium, carbon monoxide, etc.) into the region of the arc to produce optically clearer product crystals. The assembly included a removable bridging electrode for reducing interelectrode resistance during startup.

The above-mentioned powder receiver was packed with a 60-kg. charge of commercial-grade magnesium oxide (bulk density, 0.64.0 g./cm. cost, a $9/kg.). The electrodes then were energized at a power level of 10 kw. for a period of 8 minutes, at which time the bridging electrode was withdrawn from the assembly, leaving a fissure for egress of combustion gases and other gases. During the next 120 minutes the are power increased to a value of about 35 kw., and throughout the next 90 minutes was maintained at a value in the range of about 34 to 36 kw. At the end of this period of electrodes were deenergized and the commercial-grade melt (4, FIG. 2) was permitted to cool. A port then was carefully formed in the compacted powder wall to provide access to the cavity formed by melting, and a S-kg. crystalline mass was removed in large pieces for harvesting of the individual crystals by cleavage. The harvested crystals were typical of those prepared by standard submerged-arc fusion.

Second Melting Operation Following removal of the -kg. mass and the insertion of new electrodes, the cavity and port in the commercial-grade oxide compact were packed with 15 kg. ofa highly pure magnesium oxide (cost, z $1,000/kg.) whose bulk density had been increased to 1.3 g./cm. by isostatic compaction. The electrodes then were energized at a power level of kw. for 10 minutes, and the bridging electrode removed. The parameters for this second melting operation were controlled to melt a somewhat smaller volume of oxide powder. That is, during the [20 minutes followingremoval of the bridging electrode, the power input increased to 32 kw. and was maintained at this value for the remaining 60 minutes of the run. Throughout this second melting operation, argon sweep gas was passed through the electrodes and into the region of the are at a total flow rate ofO. l-O.3 std. l/sec.

After a cooling period, a port was carefully formed in the powder compact, and a 5-kg. crystalline mass was removed from the cavity formed by melting. The single crystals subsequently harvested by cleavage had a purity comparable with that of the more-valuable oxide starting material. The typical harvested crystal was about 3 cm. wide and 5 cm. long. Owing to the use of the sweep gas, the typical crystal exhibited a higher degree of optical clarity than Mg0 crystals produced under otherwise similar conditions.

Following removal of the mass of high-purity crystals from the compacted-powder containment vessel, the interior of the cavity (7, FIG. 5) was examined. The wall of the cavity was lined with a continuous, nonporous sintered layer of the highpurity magnesium oxide. The thickness of this layer averaged about 1 cm.

Cost

In terms of the cost of the powder required for crystal production, the 5-kg. mass of crystals obtained in the first melting operation cost $108/kg. The corresponding cost of the mass of high-purity crystals in the second melting operation was 3,000/kg., since only additional kilograms of powder (at $1,000/kg.) was required for this operation. Had the highpurity crystals been produced by the conventional submergedarc fusion techniquei.e., by conducting the melt in a single powder pack consisting entirely of the more-valuable oxidethe unit cost would have been $12,000/kg.

in the form of the invention just described, both the outer pack 1 and inner pack 6 are referred to as consisting essentially of magnesium oxide. As used herein, consisting essentially of" refers to powders or powder packs composed mainly of the material specified but also including normally present impurities and, where applicable, minor percentages of various doping agents-such as chromium oxide, nickel oxide, copper oxide, heavy water, and erbium oxide-provided to modify proper ties of the predominant material.

In the foregoing example, the establishment of an arc in the outer pack 1 is described as being accompanied by the accumulation of a melt 4 in the resulting cavity 3. The formation of such a melt is not an essential part of this invention, however, and it should be understood that the properties of the outer pack may be such that the cavity 3 is formed by vaporization and not melting. For example, if the outer pack consists essentially of magnesium oxide (or some other suitable outer pack material) having a bulk density below about 25 percent of its crystalline-state density, formation of the cavity 3 may be accomplished essentially by vaporization. Thus, this process encompasses the alternatives of forming or not forming a melt in the first cavity-forming operation. if a melt is so produced, it is possible to let the resulting crystalline mass remain in the cavity 3 and then to form the powder pack 6 within the cavity 3 as described. Good product crystals can be obtained by this procedure. However, it is preferable to remove the solidified melt 4, as in the preceding example.

in accordance with this invention the outer powder pack 1 is formed of refractor sinterable material. Such material serves as a suitable containment vessel and when used in sufficient quantity provides the necessary thermal and electrical insulation. As desired, the outer pack 1 may be formed of one or more of a wide variety of materials, a few examples of which follow: Ba0, CaO, Mg0, Sr0, MgC0 CaCO SrCO BaC0 M gC 0 0, and Ca(OH Referring now to thinner pack 6, melting of this pack is desired. Thus, the material for this pack is selected to consist essentially of refractory sinterablematerial having a bulk density which is at least 30 percent of its crystalline-state density. Other requirements for this pack are that it should have a vapor pressure at the melting point of less than one atmosphere and that it form essentially stoichiometric crystals on cooling from the molten state. Because, in the present process, the melt formed in the inner pack is isolated from the outer pack, the two packs may be composed of dissimilar compounds meeting the above-mentioned criteria for the respective packs. in general, the use of dissimilar materials forming eutectics is avoided unless, of course, this does not interfere with formation of the desired product crystals.

In accordance with this invention, the second powder packi.e., the inner pack from which crystals are derived in the second cavity-forming operation-may be formed in a single step or a plurality of steps. As described, it may be formed by packing the first cavity 3 with refractory sinterable powder meeting the above-mentioned criteria for the inner pack. Alternatively, it may be formed by (l) packing the first cavity with powder which is a precursor for a compound meeting the above-mentioned criteria for the inner pack and (2) heating the packed precursor in situ and in the presence of selected gases to convert the precursor to that compound. For example, the first cavity 3 may be packed with calcium carbonate, and the packed powder subsequently heated to convert the powder to calcium oxide. The following are a few examples of other suitable precursor powders: SrC0 MgC0 BaCO SrC 0 0, MgC 0 '2H 0, BaC 0 Ca(Ol-l) and Ba(0H),. Using conventional technology, these precursors may be converted, respectively, to powder packs of Sr0, Mg0, Ba0, Sr0, Mg0, Ba0, CaO, and BaO. Other examples of suitable precursors include formates, acetates, and nitrates of various alkaline-earth oxides.

In the foregoing example, this process was illustrated in terms of the production of single crystals from a highly valuable material. The process can, however, also be used to advantage to produce single crystals from materials having a low-unit cost. This can be illustrated in terms of the production of crystals of commercial-grade Mg0. Consider the case where, using conventional techniques, a S-kg. mass of crystals is formed by arc-melting a cavity in a 60kg. pack of commercial-grade powder costing $9/kg. The pack then is broken away to permit recovery of the 5 kg. of the crystalline materia], whose unit cost is thus $l08/kg. If additional material is desired, similar runs are conducted, each characterized by a product cost of $l08/kg. Now consider the case where the present process is utilized to produce the same kind of material. An arc is established in the 60-kg. pack to form a first cavi ty containing a melt. The resulting 5 kg. of crystalline material is removed through a carefully formed port, at a unit cost of l08/kg. Additional commercial-grade MgO (e.g., kg.) then is packed in the cavity, after which a second run is conducted. In this second run, however, melting parameters are controlled to ensure the formation of a shell 8, or barrier, of kind previously described. At the end of this run, a port is formed and essentially 5 kg. of crystalline material is recovered-at a cost of $27/kg. An indefinite number of runs similar to the second may then be conductedeach with essentially the same yield and the same unit cost.

It is to be understood that the foregoing discussion of our invention is presented for the purpose ofillustration and that the scope of our invention is limited only by the following claims.

What is claimed is:

l. The method of producing single crystals comprising:

a. forming a first powder pack of refractory sinterable material;

b. establishing an electric arc within said first powder pack for the time sufficient to melt only an internal portion thereof and to generate a first internal cavity therein;

c. providing in the wall ofsaid first powder pack a port communicating with said first cavity;

d. forming, within said first cavity, a refractory and sinterable second powder pack consisting essentially of material characterized by a bulk density which is at least 30 percent of its crystalline-state density, by a vapor pressure at its melting point of less than one atmosphere, and by formation of essentially stoichiometric crystals on cooling from the molten state;

e. establishing an electric arc within said second powder pack for a time sufficient to melt only an internal portion thereof, thus generating a second internal cavity containing molten material derived from said second powder pack;

f. tenninating the melting operation to cool and crystallize said molten material;

g. selectively recovering the crystallized material; and

h. harvesting single crystals from the recovered crystallized material.

2. The method of claim 1 wherein molten material collect ing in said first cavity during step (b) is permitted to cool and crystallize, and the resulting crystallized material is removed through said port prior to formation of said second powder pack.

3. The method of claim 1 wherein said second powder pack is formed by packing said first cavity with a refractory sinterable powder having the characteristics set forth in step (d).

4. The method of claim 1 wherein said second powder pack is formed by packing said first cavity with a powdered precursor of a refractory sinterable compound having the characteristics set forth in step (d) and then heating the packed precursor in situ in a selected atmosphere to convert said precursor to said compound.

5. The method of claim 1 wherein said first powder pack and said second powder pack are composed of powders having the same chemical formula.

6. The method co claim 1 wherein said first powder pack and said second powder pack respectively consist essentially of two different chemical compounds which when heated together do not form a eutectic.

7. The method of claim 1 wherein said first powder pack and said second powder pack each consist essentially of an alkaline earth oxide.

8. The method of claim 7 wherein said first powder pack and said second powder pack consist of material having the same chemical formula.

9. The method of claim 8 wherein said first powder pack and said second powder pack consist essentially of magnesium oxide. 

2. The method of claim 1 wherein molten material collecting in said first cavity during step (b) is permitted to cool and crystallize, and the resulting crystallized material is removed through said port prior to formation of said second powder pack.
 3. The method of claim 1 wherein said second powder pack is formed by packing said first cavity with a refractory sinterable powder having the characteristics set forth in step (d).
 4. The method of claim 1 wherein said second powder pack is formed by packing said first cavity with a powdered precursor of a refractory sinterable compound having the characteristics set forth in step (d) and then heating the packed precursor in situ in a selected atmosphere to convert said precursor to said compound.
 5. The method of claim 1 wherein said first powder pack and said second powder pack are composed of powders having the same chemical formula.
 6. The method co claim 1 wherein said first powder pack and said second powder pack respectively consist essentially of two different chemical compounds which when heated together do not form a eutectic.
 7. The method of claim 1 wherein said first powder pack and said second powder pack each consist essentially of an alkaline earth oxide.
 8. The method of claim 7 wherein said first powder pack and said second powder pack consist of material having the same chemical formula.
 9. The method of claim 8 wherein said first powder pack and said second powder pack consist essentially of magnesium oxide. 